In general, an X-ray tube uses a thermoelectron source as an electron source, forms a desired electron beam by accelerating and focusing thermoelectrons emitted from a filament heated to a high temperature, and generates X-rays by irradiating an X-ray target made of a metal with the electron beam.
Recently, there have been developed, as an electron source replacing this thermoelectron source, a Spint-type electron source which extracts electrons by applying high electric fields to the tips of fine needles or a cold cathode type electron source using carbon nanotubes and the like. As these applications of these techniques, techniques of extracting a single electron beam are described in patent references 1, 2, and 3:
Patent Reference 1: Japanese Patent Laid-Open No. 08-264139
Patent Reference 2: Japanese Patent Laid-Open No. 09-180894
Patent Reference 3: Japanese Patent Laid-Open No. 2005-237779
It is known that when X-rays are generated by irradiating a target with an accelerated electron beam, the generation efficiency of X-rays is very low. Most of the energy of an electron beam applied to the target is converted into heat. For this reason, a conventional X-ray generator uses a rotating anode structure in which a target is rotated.
In a structure in which an electron beam irradiation surface faces an X-ray generation surface as described in patent reference 1, it is difficult to use a rotating anode which suppresses the generation of heat as described in patent reference 3.
Medical diagnosis equipment or the like requires a certain amount of current to secure image quality necessary for diagnosis. In order to obtain high sharpness for an image, it is preferable that the size of X-ray focus formed on a target be small. However, as the size of an X-ray focus is decreased while a certain amount of current is secured, a heat load is focused on a small area, thus causing the above problem of the generation of heat. On the other hand, as the size of an X-ray focus is increased, the sharpness decreases.
It is an object of the present invention to provide an X-ray generator and X-ray imaging apparatus which achieve a large current in the generation of X-rays without decreasing the sharpness of an image.